1. Technical Field
Example embodiments relate to memory devices, and more particularly to phase change memory devices and memory systems including the same.
2. Description of the Related Art
Nonvolatile memory devices using resistance materials include a resistive random access memory (RRAM), a phase change random access memory (PRAM), and a magnetic random access memory (MRAM). Dynamic random access memory (DRAM) and flash memory devices store data using charges. Nonvolatile memory devices using resistance materials store data using the resistance of variable resistive elements (e.g., RRAM), phase change of phase change material such as chalcogenide alloy (e.g., PRAM), and resistance change of magnetic tunnel junction (MJT) thin films according to the magnetization state of a ferromagnetice substance.
Using phase change memory cells as an example, the phase change material changes into a crystalline state or an amorphous state by cooling after heating. Since the phase change material in the crystalline state has a low resistance and the phase change material in the amorphous state has a high resistance, the crystalline state may be defined as set data (0), and the amorphous state may be defined as reset data (1).
Sensing margin is very important for securing credibility of read operation of the phase change memory device because the sensing margin is a criterion for discriminating a difference of resistance distributions of the set data and the reset data which are determined after write operation is performed. When the sensing margin narrows due to various factors, such as temperature, processes associated with manufacturing the phase change memory device and operating power supply voltage provided to the phase change memory device, the set state and the reset state may not be correctly distinguished. Then, the credibility of operations of the phase change memory device is degraded and yield of the phase change memory device may be reduced.